1. Field of the Invention
This invention pertains to the general field of optical filters and, in particular, to a tunable optical filter with improved spectral performance and undeviated pass and stop beams.
2. Description of the Prior Art
Optical filters are widely used as components of optical systems. In particular, they are employed in optical communication systems, where information is transmitted along the same optical path at different wavelengths λ of light (channels). In order to retrieve information contained in a particular channel, the signal wavelengths have to be spectrally separated. Similarly, in order to add a particular channel to the stream of optical information, a spectral addition of a particular wavelength is often required. These functions have been traditionally accomplished with the use of optical add/drop modules (OADMs, also generally known as optical multiplexers and de-multiplexers) performing what is known in the art as wavelength division multiplexing (WDM). This is achieved in principle using various optical filters that pass at least a portion of the input light within a predetermined spectral range and reflect (stop) at least a portion of the light within another spectral range.
The application requirements of increasingly more complex optical systems are driving this technology towards more flexible and reconfigurable arrangements where the wavelengths of pass/stop channels (otherwise also referred to in the art as add/drop channels) may be varied at the request of the user. This can be accomplished by arranging the optical add/drop modules and the optical filters they are based upon in a tunable fashion.
A wide variety of tunable optical filters is known in the art, the most fundamental one being a simple thin-film filter fabricated by depositing a thin-film stack on a suitable substrate. Almost all thin-film optical coatings depend, at least in part, on interference for their operation. Therefore, the spectral characteristics of such filters are determined by the mixture of intrinsic optical properties of the filter materials (such as reflectance, transmittance, absorptance) and by their geometric arrangement (thickness, for instance).
Spectral tuning of a filtering function (or spectral shifting of the peak wavelength of the pass/stop bands of a filter) in such a tunable filter can be provided merely by varying the angle of incidence of the input light beam (defined and measured with respect to the normal to the surface of the filter). As is well known in the art, variation of the angle of incidence of the input light is most easily accomplished by the physical rotation of the thin-film filter with respect to the incident light, as shown in FIG. 1. For convenience, a system of Cartesian coordinates is provided as a reference throughout this disclosure.
The conventional tunable thin-film optical filter 1 illustrated in FIG. 1 is composed of a suitable, transparent, filter substrate 12 with optical quality front and back surfaces, 14 and 16, respectively, that are parallel to each other, and of a thin-film structure F. Thus, the substrate 12 is a plane-parallel optical plate. The thin-film structure F is deposited on at least part of the front surface 14 of the substrate 12 using one of well-known techniques, for example electron-beam deposition. The back surface 16 of the substrate 12 may be appropriately AR-coated to suppress residual reflections. The filter 1 may be positioned in air.
A generally non-monochromatic collimated input beam of light I is delivered toward the front surface 14 along the z axis at an oblique angle of incidence θ. Some spectral components of the beam I are transmitted through the thin-film filter 1 according to its spectral characteristics and produce the pass beam P. Some other spectral components of the beam I are reflected by the filter 1 in the form of the stop beam S. As shown in FIG. 1, both the pass and stop beams are shifted with respect to the input beam I. Those skilled in the art readily understand that the pass beam P is laterally shifted with respect to the incident beam I by a distance that depends on the thickness d of the substrate and the refractive index n. Similarly, the stop-beam S is angularly deviated with respect to the input beam I according to Snell's law.
In a thin-film narrow-band filter, the peak wavelengths of its pass band and stop band are proportional to the effective phase thickness of the filter. A filter rotation around an axis that is parallel to the x-axis, as shown in FIG. 1, from the initial angle of incidence θ to another angle of incidence θ′ changes the effective phase thicknesses that various spectral components of the incident light experience upon traversing the thin-film stack. This, in turn, produces the desired spectral shift (tuning) of the filter characteristics and yields new pass and stop beams, P′ and S′, respectively. For small rotation angles, this is the principal effect.
However, several complicating effects limit the degree to which this tuning mechanism can be utilized in a practical optical system. First, at oblique angles of incidence, the filter function is strongly dependent on the state of polarization of the incident light. Such polarization dependence detrimentally affects the performance of most optical systems and thus is extremely undesirable. Further, at oblique incidence and larger rotation angles, even when the polarization state of the incident light is well controlled, the change in optical admittances of thin-film layers comprising the filter starts to affect the effective phase thickness of the filter. In practice, the performance contribution of high-index layers is affected less than that of low-index ones, thereby causing a split of filtering characteristic for a given plane of polarization, with attendant degradation of performance. Finally, due to the angular rotation of the filter, the direction of the reflected beam (the stop channel S in FIG. 1) is constantly changing according to Snell's law, as is the transverse shift of the transmitted beam (the pass channel P in FIG. 1). This necessarily complicates the basic system design required to achieve an accurate detection of the pass and stop channels of an optical system.
To the extent that tilting or rotation of the thin-film filter is necessary to the function of the tunable filter, these drawbacks are unavoidable as long as the incident light is not linearly polarized, and as long as the useful outputs (the pass and stop beams) are collected in separate reflected and throughput paths of the apparatus. Thus, there remains a need for a tunable filter that overcomes the severe limitations described above.